If you don't have Minecraft then you don't need to kill it. Terraria is less money anyway.

If you do unfortunately have Minecraft installed, then right click the launcher and click the delete button. Then, clear it from your recycling bin and be done with it forever.

If deleting MC doesn't work, try summoning the WoF (I'll teach you how later if you don't know) and throwing a dirt block in front of it. The WoF will eat the dirt block and literally consume minecraft entirely. Minecraft will implode and fold in on itself. Bam, no more minecraft.

If the two methods above don't work, just cry. Minecraft won't go away and you're stuck with it.